Was the item larger than a letter size? Then on your proof of posting at the bottom is a 16 digit long number.
It's not tracking number, but it can show if an item has been delivered, or attempted delivery made, this counts in eBay's eyes too, or no delivery made.
Enter the number into Royal Mail's Track & Trace online to see the item's progress.
If it has shown delivery or attempted delivery, entering the number into any case the buyer opens will close it in yor favour, no need to refund.
You can send the buyer the number for them to check themselves, the item could be sitting with a neighbour.
Enter those numbers now, where you marked the item as dispatched where it says Add tracking, these numbers are valid too.
@riosbargains